Objection 1. It would seem that passion is in the apprehensive part of the soul rather than in the appetitive. Because that which is first in any genus, seems to rank first among all things that are in that genus, and to be their cause, as is stated in Metaph. ii, 1. Now passion is found to be in the apprehensive, before being in the appetitive part: for the appetitive part is not affected unless there be a previous passion in the apprehensive part. Therefore passion is in the apprehensive part more than in the appetitive.
Objection 2. Further, what is more active is less passive; for action is contrary to passion. Now the appetitive part is more active than the apprehensive part. Therefore it seems that passion is more in the apprehensive part.
Objection 3. Further, just as the sensitive appetite is the power of a corporeal organ, so is the power of sensitive apprehension. But passion in the soul occurs, properly speaking, in respect of a bodily transmutation. Therefore passion is not more in the sensitive appetitive than in the sensitive apprehensive part.
On the contrary, Augustine says (De Civ. Dei ix, 4) that "the movement of the soul, which the Greeks called pathe, are styled by some of our writers, Cicero [Those things which the Greeks call pathe, we prefer to call disturbances rather than diseases (Tusc. iv. 5)] for instance, disturbances; by some, affections or emotions; while others rendering the Greek more accurately, call them passions." From this it is evident that the passions of the soul are the same as affections. But affections manifestly belong to the appetitive, and not to the apprehensive part. Therefore the passions are in the appetitive rather than in the apprehensive part.
I answer that, As we have already stated (1) the word "passion" implies that the patient is drawn to that which belongs to the agent. Now the soul is drawn to a thing by the appetitive power rather than by the apprehensive power: because the soul has, through its appetitive power, an order to things as they are in themselves: hence the Philosopher says (Metaph. vi, 4) that "good and evil," i.e. the objects of the appetitive power, "are in things themselves." On the other hand the apprehensive power is not drawn to a thing, as it is in itself; but knows it by reason of an "intention" of the thing, which "intention" it has in itself, or receives in its own way. Hence we find it stated (Metaph. vi, 4) that "the true and the false," which pertain to knowledge, "are not in things, but in the mind." Consequently it is evident that the nature of passion is consistent with the appetitive, rather than with the apprehensive part.
Reply to Objection 1. In things relating to perfection the case is the opposite, in comparison to things that pertain to defect. Because in things relating to perfection, intensity is in proportion to the approach to one first principle; to which the nearer a thing approaches, the more intense it is. Thus the intensity of a thing possessed of light depends on its approach to something endowed with light in a supreme degree, to which the nearer a thing approaches the more light it possesses. But in things that relate to defect, intensity depends, not on approach to something supreme, but in receding from that which is perfect; because therein consists the very notion of privation and defect. Wherefore the less a thing recedes from that which stands first, the less intense it is: and the result is that at first we always find some small defect, which afterwards increases as it goes on. Now passion pertains to defect, because it belongs to a thing according as it is in potentiality. Wherefore in those things that approach to the Supreme Perfection, i.e. to God, there is but little potentiality and passion: while in other things, consequently, there is more. Hence also, in the supreme, i.e. the apprehensive, power of the soul, passion is found less than in the other powers.
Reply to Objection 2. The appetitive power is said to be more active, because it is, more than the apprehensive power, the principle of the exterior action: and this for the same reason that it is more passive, namely, its being related to things as existing in themselves: since it is through the external action that we come into contact with things.
Reply to Objection 3. As stated in the I, 78, 3 the organs of the soul can be changed in two ways. First, by a spiritual change, in respect of which the organ receives an "intention" of the object. And this is essential to the act of the sensitive apprehension: thus is the eye changed by the object visible, not by being colored, but by receiving an intention of color. But the organs are receptive of another and natural change, which affects their natural disposition; for instance, when they become hot or cold, or undergo some similar change. And whereas this kind of change is accidental to the act of the sensitive apprehension; for instance, if the eye be wearied through gazing intently at something or be overcome by the intensity of the object: on the other hand, it is essential to the act of the sensitive appetite; wherefore the material element in the definitions of the movements of the appetitive part, is the natural change of the organ; for instance, "anger is" said to be "a kindling of the blood about the heart." Hence it is evident that the notion of passion is more consistent with the act of the sensitive appetite, than with that of the sensitive apprehension, although both are actions of a corporeal organ.
Source: Summa Theologica (New Advent)